There is known a method of polymerizing an olefin wherein a metallocene compound having a group having conjugated π electrons particularly cyclopentadiene and its derivative as a ligand is used as a catalyst to be combined with an organoaluminum oxy compound or an organoboron compound. For example, JP-A 58-19309 discloses a method of polymerizing an olefin wherein bicyclopentadienyl zirconium dichloride and methyl aluminoxane are used as a catalyst. Recently, a system wherein olefin polymerization proceeds without using combination with an organoaluminum oxy compound or an organoboron compound is also disclosed. For example, JP-A 4-211405 discloses a method of polymerizing an olefin by using a combination of a zirconium metallocene compound and a solid carrier obtained by contacting magnesium chloride and an active hydrogen-free electron donor with active hydrogen-containing electron donor. Further, JP-A 7-330821 discloses a method of polymerizing an olefin by using a combination of a titanium metallocene compound and a solid component obtained by reacting a magnesium compound such as magnesium halide with an organoaluminum compound.
However, a majority of known methods including the method of JP-A 58-19309 suffer from a problem that the metallocene compound when used without combination with an expensive organoaluminum oxy compound or organoboron compound cannot exhibit a high olefin polymerization activity. Even if the metallocene compound exhibits a high olefin polymerization activity by combination with an expensive organoaluminum oxy compound or organoboron compound, the duration of the activity was often short. Further, the bulk density of a formed polymer after polymerization is low to make handling difficult, and when such catalysts are applied to a gaseous phase or liquid phase polymerization method, the resulting polymer adheres to a polymerizer wall thus deteriorating heat transfer to make cooling difficult and to permit formation of polymer agglomerates, and thus there are many problems to be solved in process in order to use such catalysts in producing polyolefin in an industrial scale.
In recently disclosed methods of permitting olefin polymerization to proceed without combination with an organoaluminum oxy compound or an organoboron compound, on one hand, there still remain many features to be improved from an economical point of view and from the viewpoint of large-scale production. In JP-A 4-211405 supra, for example, dibutyl magnesium which is expensive and unstable in the air should be used as a starting material in order to regulate the particle diameter of magnesium chloride as a carrier, and in JP-A 7-330821 supra, expensive and unstable diethoxy magnesium is used as a carrier. It is hardly said that these known techniques not using combination with an organoaluminum oxy compound or an organoboron compound are satisfactory in respect of the polymerization activity, the powdery properties of a formed polymer, and molecular weight characteristics such as molecular-weight distribution etc.